1. Field of the Invention
The present invention relates to time recording devices and methods and in particular to such time recording devices and methods which can be used to realize a currentless time normal, e.g. to permit a delay in the access time to a chip.
2. Description of Prior Art
Electronic circuits such as chipcard ICs are subject to a special requirement as regards rejecting unauthorized accesses. To prevent unauthorized accesses, electronic circuits which perform security functions, e.g. chipcard ICs, are secured by employing encryption methods. Such encryption methods can be investigated by addressing the electronic circuit with signal patterns which are changed only gradually, e.g. within the framework of a DPA (differential power analysis), the electronic circuit being switched on and off periodically in response to such attacks. A successful attack, which would make the electronic circuit or the chipcard IC vulnerable to an unauthorized access, requires a certain number of switchon events. This attack can be made more difficult or prevented by increasing the time between the switchon events or by permitting only a certain number of switchon events in a specified time interval.
To be able to take the above measures against unauthorized accesses, time recording must be provided on the electronic circuit or the chip, and this should preferably also operate in the switched-off state, i.e. without energy being supplied. In addition such time recording should be substantially temperature independent and be immune to external influences.
From U.S. Pat. No. 5,760,644 an integrated semiconductor circuit is known by means of which a time lapse can be recorded. According to U.S. Pat. No. 5,760,644 charge carriers are introduced for this purpose into a storage dielectric and changes in the electric field arising from the charge introduced into the dielectric are used for time recording. This electric field changes with increasing natural charge diminution in the dielectric material, so that measurement of the electric field with time provides information on the time which has elapsed. The storage dielectric used in U.S. Pat. No. 5,760,644 is the ONO structure (oxide-nitride-oxide structure) of an SONOS transistor (silicon-ONO-silicon transistor) or the NO structure (nitride-oxide structure) of an SNOS transistor (silicon-NO-silicon transistor). Using the ONO dielectric or the NO dielectric of the above storage transistors, the time-dependent charge loss of the electrons stored in the nitride is established through the altered effective voltage of the cell.